Changed composer.json to work with newer symfony and without mongodb

Fixed version string for ```symfony/framework-bundle```

I removed the unnnessecary Zend modules and moved them into
```require-dev```, since I couldn't find any references to the
```DoctrineModule``` or ```DoctrineORMModule``` inside the source
code.

I moved the requirements for ```doctrine/mongodb``` and
```doctrine/mongodb-odm``` into ```require-dev``` and ```suggest```,
so a notice is displayed if someone wants to use it with ODM.

Also added the ```dev``` flag to ```composer install``` in ```.travis.yml```